#18113: avoid segfault if Py_XDECREF triggers code that calls set_panel_userptr again

Problem noted & original patch by Serhiy Storchaka; I tweaked the patch a bit.
diff --git a/Lib/test/test_curses.py b/Lib/test/test_curses.py
index 387a185..fcf9618 100644
--- a/Lib/test/test_curses.py
+++ b/Lib/test/test_curses.py
@@ -262,6 +262,14 @@
     if sys.getrefcount(obj) != nrefs:
         raise RuntimeError, "set_userptr leaked references"
 
+def test_userptr_segfault(stdscr):
+    panel = curses.panel.new_panel(stdscr)
+    class A:
+        def __del__(self):
+            panel.set_userptr(None)
+    panel.set_userptr(A())
+    panel.set_userptr(None)
+
 def test_resize_term(stdscr):
     if hasattr(curses, 'resizeterm'):
         lines, cols = curses.LINES, curses.COLS
@@ -281,6 +289,7 @@
         window_funcs(stdscr)
         test_userptr_without_set(stdscr)
         test_userptr_memory_leak(stdscr)
+        test_userptr_segfault(stdscr)
         test_resize_term(stdscr)
         test_issue6243(stdscr)
     finally: